In the field of polymer chemistry, the search continues for new compounds suitable as monomers for the preparation of polymers useful for a variety of purposes. New routes to known monomers also are valuable in providing alternative methods of producing valuable monomers, such as pyromellitic anhydride which is particularly useful in producing thermally stable heterocyclic polymers such as described in 16 Encycl. Chem. Tech., 42 (1968).